Squid and other cephalopods have the ability to change skin color using muscular chromatophore organs that are under direct neural control. All work on the cellular mechanisms of chromatophore control in squid has focused on three species in the family Loliginidae that inhabit coastal environments rich in benthic features like seaweed, rocks and coral. Skin-color changes in these species are associated with camouflage as well as intra-specific signaling and deimatic displays. The open ocean presents a radically different environment that is also inhabited by many squids, primarily of the family Ommastrephidae, one that includes the Humboldt squid (Dosidicus gigas). There is little light in the oceanic water column at depths inhabited by these squid during daytime, and static visual features are non-existent. Novel color-change behaviors in Dosidicus include repetitive whole-body “flashing,” used for intra-specific signaling, and chaotic-like “flickering” that may underlie crypsis in the open ocean. Although these dynamic behaviors contrast with the generally more static patterns in loliginids, squids of both families employ temporal and spatial patterning to varying degrees. It is therefore likely that basic mechanisms for controlling the chromatophore network are shared by most, if not all, squids. Descending “vertical” control from the brain to the chromatophore musculature is well established in loliginids and may account for most chromogenic behaviors in those species, but behaviors in ommastrephids like flickering may be more influenced by processes within the skin itself that permit excitation to spread from one chromatophore to another without directly involving the nervous system. This hypothetical pathway would define a “horizontal” or distributed control system in the periphery that would permit autonomous behavior within the chromatophore network. Horizontal control is relevant to the vascular bed, gut and coupled neural micro-circuits in vertebrates, and results from this project will thus influence this broader field. From a wider perspective, results of this project will be relevant to interactions of distributed (horizontal) and top-down (vertical) control mechanisms, an inherent feature of complex systems to generate non-predictable, emergent phenomena. This concept is of fundamental interest to a broad sector of society, ranging from engineering to economics to politics.
An integrated approach will be followed to test the hypothesis that control of the chromatophore network in squid involves peripheral mechanisms that are distinct from the neuronal motor-control pathway that descends from the brain. Spontaneous chromatophore activity that is independent of canonical neural control will be isolated by experimental manipulations in loliginid squid (Doryteuthis opalescens), including chronic denervation and pharmacological block of neuronal activity with tetrodotoxin. In addition, a comparative approach will take advantage of an ommastrephid species, Dosidicus gigas, in which spontaneous, tetrodotoxin-resistant chromatophore activity is extremely prominent. Relevant methods involve cellular electrophysiology, molecular transcriptomics, immunohistochemistry with confocal microscopy and high-resolution electron microscopy. Specific aims are: 1) identify molecular and physiological properties of relevant ion channels and receptors that control excitability in the radial muscle fibers that operate individual chromatophore organs in Doryteuthis; 2) define structural, molecular and physiological features of coupling mechanisms between muscle fibers of neighboring chromatophores that define an excitatory transmission pathway within the skin; 3) elucidate the inhibitory role in controlling spontaneous chromatophore activity played by serotonin; 4) carry out parallel experiments in Dosidicus, a member of a family of ecologically important squid in which cellular studies of chromatophores have never been carried out.
Hopkins Marirne Station and Santa Rosalia, Baja California Sur, Mexico
- Josh Rosenthal, Professor, University of Puerto Rico
- Bill Kier, Professsor, University of North Carolina, Chapel Hill